Conventionally, polyurethane in powder form is prepared by the process of mechanical grinding of solid polyurethane (JP-02-124978). This process generates heat which needs to be removed by external cooling using liquid nitrogen at cryogenic temperatures. So this method does not yield polyurethane powder in controlled particle size and shape.
Polyurethane powder lacquers are prepared by first blocking polyisocyanates with a suitable blocking agents such as--caprolactam (DE 4,134,032) or 2,5-diketopiperazines (DE 4,204,995). The blocked polyisocyanates are then mixed with a prepolymer containing hydroxyl groups and other components such as pigment and fillers, homogenized at 80.degree.-120.degree. C. in an extruder. After cooling, the extrudate is broken and ground mechanically to yield powder polyurethane. JP 03-181526 disclose a method for preparation of polyurethane microparticles which involves dissolving polyurethane resin in amino alcohol at 130.degree.-150.degree. C. followed by cooling, filtering and drying under vacuum.
JP 04-248875 teaches a method for the preparation of polyurethane powder by reacting a resin having hydroxyl and/or amino group with blocked isocyanates in a suitable solvent at elevated temperature. Thereafter the solvent is removed by lowering the pressure. The powder is filtered and dried.
Spherical polyurethane particles are prepared by dispersion polymerization using water and/or alcohol as dispersing medium (JP 04-161416). Polyisocynate, polyol and other additives/reagents are first dissolved in organic solvent which is dispersed in water in presence of a stabilizer such as poly(ethyleneoxide- propyleneoxide) block copolymer, gelatin, poly(vinylalcohol), methyl cellulose or sodium alkyl sulfate.
In another variant of this process disclosed in JP 04-76016, urethane prepolymer along with a protective colloid such ethylene oxide propylene oxide block copolymer is dispersed in water containing a surfactant such a sodium salt of polyacrylic acid. Addition of bridging agent such a polyamine and stirring the mixture at room temperature yields polyurethlaiie powder. This technique has been employed in the preparation of polyurethane micocapsules containing oil soluble agent such as pesticide DDVP (Choi K. V. and Min K. S. Polymer (Korea), 14(4), 1990, pp 392-400), fenitrothion (Fuyama G.et. al., J. Pesticide Sci. 9, 1984, pp 511-516) and a non steroidal antiinflammatory agent diclofenac (Siva Readdy P. V. et. al., Macromolecular Reports, A32 (suppls 5&6)., 1995, pp 789-799). The principal drawback of these methods is that one obtains polydisperse particle sizes in the range of 50 to 500 microns.
Japanese Patents Nos. 04-255755,03-31359 and 04-202331 describe the use of a stabilizer derived from polycaprolactone polybutadiene and isophorone diisocynate for the dispersion polymerization of a polyol with a polyurethane prepolymer in n-heptane as the medium. The process was reported to give polyurethane powder without any agglomeration.
The prior art describes other stabilizers derived from poly(butylene adipate), maleic anhydride and lauryl methacrylate (JP 02-38453) and an addition product of alkoxy polyethylene-polypropylene glycol and polyisocyanates (JP 01-165617). Both these stabilizers can be used for the dispersion polymerization of polyurethane in a non aqueous medium. In these prior art processes, the proportion of stabilizer used is very high (10-30 wt % of total quantity of polyurethane forming reactants). This produces polydisperse particles having size range between 1-1000 microns. Furthermore, the reactions are carried out at elevated temperatures i.e. between 100.degree.-200.degree. C.
Particle forming polymerization processes are well known for vinyl monomers in the prior art, whereas particle forming polycondensation. (addition) processes are far less developed (Dispersion Polymerization in Organic medium, Ed. Barret K. E. J., John Wiley & Sons, 1975). Particle forming polycondensation processes in non-aqueous medium is even less rare. A stable dispersion of droplets or particles result when the attractive potential between two particles is less than repulsive potential. As repulsive potential is directly proportional to the dielectric constant of dispersion medium, stable disperaion can not be easily achieved in a medium of very low dielectric constant such as aliphatic hydrocarbons.